Customized polymer/glass diffractive waveguide stacks for augmented reality/mixed reality applications

ABSTRACT

A diffractive waveguide stack includes first, second, and third diffractive waveguides for guiding light in first, second, and third visible wavelength ranges, respectively. The first diffractive waveguide includes a first material having first refractive index at a selected wavelength and a first target refractive index at a midpoint of the first visible wavelength range. The second diffractive waveguide includes a second material having a second refractive index at the selected wavelength and a second target refractive index at a midpoint of the second visible wavelength range. The third diffractive waveguide includes a third material having a third refractive index at the selected wavelength and a third target refractive index at a midpoint of the third visible wavelength range. A difference between any two of the first target refractive index, the second target refractive index, and the third target refractive index is less than 0.005 at the selected wavelength.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No.62/865,808 filed on Jun. 24, 2019, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

This invention relates to customized polymer/glass diffractive waveguidestacks with improved performance for augmented reality/mixed realityapplications.

BACKGROUND

Augmented reality/mixed reality devices typically exploit a single typeof glass or polymer material for all layers (e.g., red (R), green (G),and blue (B) layers). The overall performance of the device can bedictated by the performance of the RGB layers. Some key performanceindicators, such as modulation transfer function (MTF), efficiency,field of view (FOV), contrast, and uniformity of an eyepiece depend onoptical properties of the individual layers. These optical propertiesinclude refractive index, yellowness index, haze, optical transmission,surface roughness, and the like. For a given material, opticalproperties such as refractive index and optical transmission are afunction of wavelength. See, for example, FIGS. 1A and 1B, which showtypical dispersion curves (refractive index versus wavelength) forpolymer waveguides, and FIG. 2, which shows typical optical absorptioncurves (optical transmission versus wavelength) for polymer waveguidematerials. Other properties such as haze and yellowness index do notdepend as heavily on wavelength. This particular relationship ofwaveguide materials with incident wavelength can impose challengesrelated to optical suitability if the refractive index and correspondingyellowness index values are above a certain threshold.

SUMMARY

In a first general aspect, a diffractive waveguide stack includes afirst diffractive waveguide for guiding light in a first visiblewavelength range and a second diffractive waveguide for guiding light ina second visible wavelength range. The first diffractive waveguideincludes a first material and having a first yellowness index and afirst refractive index at a selected wavelength, and the seconddiffractive waveguide includes a second material and having a secondyellowness index and a second refractive index at the selectedwavelength. A wavelength in the first visible wavelength range exceeds awavelength in the second visible wavelength range, the first refractiveindex exceeds the second refractive index at the selected wavelength,and the first yellowness index exceeds the second yellowness index atthe selected wavelength.

Implementations of the first general aspect may include one or more ofthe following features.

The diffractive waveguide stack of the first general aspect may includea third diffractive waveguide for guiding light in a third visiblewavelength range. The third diffractive waveguide includes a thirdmaterial and having a third yellowness index and a third refractiveindex at the selected wavelength. A wavelength in the second visiblewavelength range exceeds a wavelength in the third visible wavelengthrange, and the second yellowness index exceeds the third yellownessindex at the selected wavelength. The second refractive index may exceedthe third refractive index at the selected wavelength. The first visiblewavelength range includes red light, the second visible wavelength rangeincludes green light, and the third visible wavelength range includesblue light. The first yellowness index is less than about 1.2, thesecond yellowness index is less than about 0.8, and the third yellownessindex is less than about 0.4 at the selected wavelength.

In some cases, the first material includes a first polymer and thesecond material includes a second polymer. The first polymer and thesecond polymer may be different. In certain cases, the first materialincludes a first copolymer having a first monomer and a second monomer,and the second material includes a second copolymer having the firstmonomer and the second monomer. A ratio of the first monomer to thesecond monomer in the first copolymer may differ from a ratio of thefirst monomer to the second monomer in the second copolymer. The firstmaterial may include a first additive, and the second material mayinclude a second additive. The first additive and the second additivecan be the same, with a ratio of the first additive to the first polymerdiffering from a ratio of the second additive to the second polymer.

In some cases, the first material includes a first glass and the secondmaterial comprises a second glass. In certain cases, the first materialcomprises one of a polymer and a glass, and the second materialcomprises the other of a polymer and a glass.

In a second general aspect, a diffractive waveguide stack includes afirst diffractive waveguide for guiding light in a first visiblewavelength range, a second diffractive waveguide for guiding light in asecond visible wavelength range, and a third diffractive waveguide forguiding light in a third visible wavelength range. The first diffractivewaveguide includes a first material and having a first refractive indexat a selected wavelength and a first target refractive index at amidpoint of the first visible wavelength range. The second diffractivewaveguide includes a second material and having a second refractiveindex at the selected wavelength and a second target refractive index ata midpoint of the second visible wavelength range. The third diffractivewaveguide includes a third material and having a third refractive indexat the selected wavelength and a third target refractive index at amidpoint of the third visible wavelength range. The first visiblewavelength range corresponds to red light, the second visible wavelengthrange corresponds to green light, and the third visible wavelength rangecorresponds to blue light. A difference between any two of the firsttarget refractive index, the second target refractive index, and thethird target refractive index is less than 0.005 at the selectedwavelength.

In some implementations of the second general aspect, the selectedwavelength is 589 nm.

In a third general aspect, fabricating diffractive waveguides for awaveguide stack includes combining a first monomer and a second monomerin a first ratio to yield a first polymerizable material, casting thefirst polymerizable material in a first diffractive waveguide mold andpolymerizing the first polymerizable material to yield a firstdiffractive waveguide for guiding light in a first visible wavelengthrange, combining the first monomer and the second monomer in a secondratio to yield a second polymerizable material, and casting the secondpolymerizable material in a second diffractive waveguide mold andpolymerizing the second polymerizable material to yield a seconddiffractive waveguide for guiding light in a second visible wavelengthrange. The first diffractive waveguide has a first yellowness index anda first refractive index at a selected wavelength, and the seconddiffractive waveguide has a second yellowness index and a secondrefractive index at the selected wavelength. A wavelength in the firstvisible wavelength range exceeds a wavelength in the second visiblewavelength range, the first refractive index exceeds the secondrefractive index at the selected wavelength, and the first yellownessindex exceeds the second yellowness index at the selected wavelength.

Implementations of the third general aspect may include one or more ofthe following features.

The third general aspect may further include combining the first monomerand the second monomer in a third ratio to yield a third polymerizablematerial, casting the third polymerizable material in a thirddiffractive waveguide mold, and polymerizing the third polymerizablematerial to yield a third diffractive waveguide for guiding light in athird visible wavelength range. The third diffractive waveguide has athird yellowness index and a third refractive index at the selectedwavelength. A wavelength in the second visible wavelength range exceedsa wavelength in the third visible wavelength range, and the secondyellowness index exceeds the third yellowness index at the selectedwavelength. The second refractive index may exceed the third refractiveindex at the selected wavelength. In some cases, the selected wavelengthis 589 nm.

The details of one or more embodiments of the subject matter of thisdisclosure are set forth in the accompanying drawings and thedescription. Other features, aspects, and advantages of the subjectmatter will become apparent from the description, the drawings, and theclaims.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show typical dispersion curves for polymer waveguides.

FIG. 2 shows typical optical absorption curves for waveguide materials.

FIG. 3 shows a plot of efficiency versus yellowness index forred-green-blue (RGB) waveguides.

FIGS. 4A and 4B show RGB configurations based on three different basematerials.

FIGS. 5A and 5B show RGB configurations based on the same base materialswith different refractive indices.

FIGS. 6A-6D show RGB configurations based on combinations of glass andpolymer waveguides with different refractive indices.

FIG. 7 depicts a multi-head system for fabricating RGB layers based on atwo-part liquid resin.

DETAILED DESCRIPTION

This disclosure relates to the use of optically tuned materials forvarious color layers (e.g., RGB) in an augmented reality (AR)/mixedreality (MR) diffractive waveguide-based eyepiece to optimize theoverall optical performance of the eyepiece. The material for each colorwaveguide can be tuned for optimal optical properties (refractive index,yellowness index, transmission) according to operating wavelength.Various implementations of glass and polymer-based waveguides configuredto achieve optimal optical properties are described.

Differences in refractive indices (dispersion curve) for a givenwaveguide material at RGB wavelengths typically result in a differentfield of view (FOV) for each layer and can limit the overall FOV of awaveguide stack. In addition, materials with higher refractive indices(e.g., glass as well as polymers) tend to exhibit a greater yellownessindex (b*), which is related to the optical transmission of a waveguideand overall efficiency of an eyepiece. Above a certain value ofyellowness index (b*_(lim)), the material absorption limits the overallefficiency of an eyepiece due at least in part to light absorption bythe bulk of the waveguide. The threshold values of b*_(lim) arespectrally dependent: b*_(lim) is different for various colors (R, G, B,C, . . . ) in the order B_(TH)<G_(TH)<R_(TH). That is, red layers cantolerate higher values of b*_(lim) compared to green and blue layers.FIG. 3 shows a schematic representation of threshold b* values forvarious colors based on eye box efficiency drop as the waveguideapproaches b*_(lim), with plots 300, 302, and 304 corresponding to red,green, and blue, respectively. Thus, rather than using the samewaveguide material for all three colors, an appropriate combination ofrefractive index and corresponding b* value may be selected forindividual R, G, and B layers.

To demonstrate the dependency of eyepiece efficiency with b* andwavelength, the threshold values of b* for R, G and B polymer waveguideswere obtained by fabricating all three waveguides in LUMIPLUS LPB-1102polymer (available from Mitsubishi Gas Chemical) with refractive indexof 1.71 at 589 and starting b* of 0.3. The waveguides were then exposedto an additional UV dose to induce higher yellowness in the waveguide,and efficiencies for each color were measured as a function of b*. Thethreshold values of b* were then extracted from the plots of efficiencyversus b* for each color by locating the b* position on the plot whereefficiencies start to decline noticeably.

There are various implementations for achieving a suitable combinationof refractive index and yellowness index separately for R, G and Blayers. A first implementation employs three different waveguidematerials, each with a different base material composition. A secondimplementation employs the same base material for each waveguide andadjusts the chemical composition or synthetic conditions to alter theoptical properties. A third implementation combines glass and polymerwaveguides. For a higher operating wavelength (e.g. red color, 625 nm),a material with a higher refractive index and higher b* can be used, asthe red wavelength is not as sensitive to higher b* values, and desiredeyepiece efficiency can still be maintained due to low light absorptionby the waveguide. For a lower operating wavelength (e.g. blue color, 455nm), a material with a lower index at a selected wavelength (e.g., 589nm) and lower b* can be used, as the refractive index will be higher at455 nm, and lower b* helps to keep light absorption at the minimum andthereby promote eyepiece efficiency.

Materials with higher b* and higher refractive index are suitable forred and green waveguide layers. As such, material with a refractiveindex of 1.71 may not be an ideal choice for red and green waveguidematerials, since the refractive indices at green (530 nm) and red (625nm) wavelengths are lower than 1.75. As depicted in FIG. 3, n₁<n₂<n₃,and Bb*<Gb*<Rb*, where Bb*, Gb*, and Rb* represent b* of the blue,green, and red layers, respectively.

FIGS. 4A and 4B depict RGB waveguide stacks having RGB layers having adifferent base material for at least two of the layers. FIG. 4A depictsa RGB eyepiece stack 400 in which the red waveguide 402 is made of afirst glass having a first refractive index, and the green and bluewaveguides 404 and 406, respectively, are made of a second glass havingsecond and third refractive indices, respectively, where the second andthird refractive indices are different. In one example, the redwaveguide is made of glass having a refractive index of 1.8 at 625 nmand b*<1.2; the green waveguide is made of glass having a refractiveindex of 1.8 at 530 nm and b*<0.8; and the blue waveguide is made ofglass having a refractive index of 1.8 at 455 nm and b*<0.35. FIG. 4Bdepicts a RGB eyepiece stack 410 in which the red waveguide 412 is madeof a first polymer having a first refractive index, the green waveguide414 is made of a second polymer having a second refractive index, andthe blue waveguide 416 is made of a third polymer having a thirdrefractive index, where the first, second, and third polymers aredifferent, and the first, second, and third refractive indices aredifferent. For implementations depicted in FIG. 4B, optical polymerssuch as thiol-ene, polycarbonate, CR-39, PMMA, MR 167, MR174, and otherappropriate polymers can be exploited to form a suitable combination ofmulti-color waveguide stacks for AR/MR application.

FIGS. 5A and 5B depict waveguide stacks having waveguides with differentoperating wavelengths (e.g., R, G, B, etc.) made of the same base glassor polymer material. A high index component can be combined with thebase material, curing conditions can be selected, or both, to achieve adesired combination of refractive index and yellowness index for eachcolor. FIG. 5A depicts a RGB waveguide stack 500 having the same baseglass material for red waveguide 502, green waveguide 504, and bluewaveguide 506. FIG. 5B depicts a RGB waveguide stack 510 having the samebase polymer material for red waveguide 512, green waveguide 514, andblue waveguide 516. Each layer in the waveguide stacks of FIGS. 5A and5B has a concentration of a high index component selected to achieve asuitable refractive index and yellowness index for each layer.

FIGS. 6A-6D depict waveguide stacks fabricated with a combination ofglass and polymer waveguides with varying refractive indices. FIG. 6Adepicts a RGB eyepiece stack 600 having red polymer waveguide 602 withrefractive index n₁, green glass waveguide 604 with refractive index n₂,and blue glass waveguide 606, with refractive index n₃. FIG. 6B depictsa RGB eyepiece stack 610 having red polymer waveguide 612 withrefractive index n₁, green polymer waveguide 614 with refractive indexn₂, and blue glass waveguide 616 with refractive index n₃. FIG. 6Cdepicts a RGB eyepiece stack 620 having red polymer waveguide 622 withrefractive index n₁, green glass waveguide 624 with refractive index n₂,and blue glass waveguide 626 with refractive index n₃. FIG. 6D depicts aRGB eyepiece stack 630 red polymer waveguide 632 with refractive indexn₁, green polymer waveguide 634 with refractive index n₂, and blue glasswaveguide 636 with refractive index n₃.

Customized RGB polymer waveguides described herein can be fabricatedwith a multi-head system 700 as depicted in FIG. 7 using a two-part highindex polymer resin, with the base resins 702 and 704 used for all threewaveguides. The desired refractive and yellowness indices can beachieved by selectively controlling the relative amounts of base resins702 and 704 dispensed and combined. This approach allows rapid andefficient high volume fabrication.

In one example, as depicted in FIG. 7, base resin 702 and 704 areprovided in a 1:1 ratio to red layer mixer 706. The resulting mixture isprovided to red layer casting head 708 and used to form a red waveguidefor a RGB eyepiece stack. Base resin 702 and 704 are provided in a 1:0.8ratio to green layer mixer 710. The resulting mixture is provided togreen layer casting head 712 and used to form a green waveguide for theRGB eyepiece stack. Base resin 704 is provided to blue layer mixer 714without base resin 702. Base resin 704 from blue layer mixer 714 isprovided to blue layer casting head 716 and used to form a bluewaveguide for the RGB eyepiece stack. Different ratios of base resins702 and 704 can be used in other examples.

Although this disclosure contains many specific embodiment details,these should not be construed as limitations on the scope of the subjectmatter or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particular embodiments.Certain features that are described in this disclosure in the context ofseparate embodiments can also be implemented, in combination, in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments, separately, or in any suitable sub-combination. Moreover,although previously described features may be described as acting incertain combinations and even initially claimed as such, one or morefeatures from a claimed combination can, in some cases, be excised fromthe combination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Particular embodiments of the subject matter have been described. Otherembodiments, alterations, and permutations of the described embodimentsare within the scope of the following claims as will be apparent tothose skilled in the art. While operations are depicted in the drawingsor claims in a particular order, this should not be understood asrequiring that such operations be performed in the particular ordershown or in sequential order, or that all illustrated operations beperformed (some operations may be considered optional), to achievedesirable results.

Accordingly, the previously described example embodiments do not defineor constrain this disclosure. Other changes, substitutions, andalterations are also possible without departing from the spirit andscope of this disclosure.

What is claimed is:
 1. A diffractive waveguide stack comprising: a firstdiffractive waveguide for guiding light in a first visible wavelengthrange, the first diffractive waveguide comprising a first material andhaving a first yellowness index and a first refractive index at aselected wavelength; and a second diffractive waveguide for guidinglight in a second visible wavelength range, the second diffractivewaveguide comprising a second material and having a second yellownessindex and a second refractive index at the selected wavelength, wherein:a wavelength in the first visible wavelength range exceeds a wavelengthin the second visible wavelength range, the first refractive indexexceeds the second refractive index at the selected wavelength, and thefirst yellowness index exceeds the second yellowness index at theselected wavelength.
 2. The diffractive waveguide stack of claim 1,further comprising” a third diffractive waveguide for guiding light in athird visible wavelength range, the third diffractive waveguidecomprising a third material and having a third yellowness index and athird refractive index at the selected wavelength, wherein: a wavelengthin the second visible wavelength range exceeds a wavelength in the thirdvisible wavelength range, and the second yellowness index exceeds thethird yellowness index at the selected wavelength.
 3. The diffractivewaveguide stack of claim 2, wherein the second refractive index exceedsthe third refractive index at the selected wavelength.
 4. Thediffractive waveguide stack of claim 2, wherein the first visiblewavelength range comprises red light, the second visible wavelengthrange comprises green light, and the third visible wavelength rangecomprises blue light.
 5. The diffractive waveguide stack of claim 2,wherein the selected wavelength is 589 nm, and the first yellownessindex is less than about 1.2, the second yellowness index is less thanabout 0.8, and the third yellowness index is less than about 0.4.
 6. Thediffractive waveguide stack of claim 1, wherein the first materialcomprises a first polymer and the second material comprises a secondpolymer.
 7. The diffractive waveguide stack of claim 6, wherein thefirst polymer and the second polymer are different.
 8. The diffractivewaveguide stack of claim 6, wherein the first material comprises a firstcopolymer comprising a first monomer and a second monomer and the secondmaterial comprises a second copolymer comprising the first monomer andthe second monomer.
 9. The diffractive waveguide stack of claim 6, wherea ratio of the first monomer to the second monomer in the firstcopolymer differs from a ratio of the first monomer to the secondmonomer in the second copolymer.
 10. The diffractive waveguide stack ofclaim 6, wherein the first material comprises a first additive and thesecond material comprises a second additive.
 11. The diffractivewaveguide stack of claim 10, wherein the first additive and the secondadditive are the same, and a ratio of the first additive to the firstpolymer differs from a ratio of the second additive to the secondpolymer.
 12. The diffractive waveguide stack of claim 1, wherein thefirst material comprises a first glass and the second material comprisesa second glass.
 13. The diffractive waveguide stack of claim 1, whereinthe first material comprises one of a polymer and a glass, and thesecond material comprises the other of a polymer and a glass.
 14. Thediffractive waveguide stack of claim 1, wherein the selected wavelengthis 589 nm.
 15. A method of fabricating diffractive waveguides for awaveguide stack, the method comprising: combining a first monomer and asecond monomer in a first ratio to yield a first polymerizable material;casting the first polymerizable material in a first diffractivewaveguide mold and polymerizing the first polymerizable material toyield a first diffractive waveguide for guiding light in a first visiblewavelength range, wherein the first diffractive waveguide has a firstyellowness index and a first refractive index at a selected wavelength;combining the first monomer and the second monomer in a second ratio toyield a second polymerizable material; and casting the secondpolymerizable material in a second diffractive waveguide mold andpolymerizing the second polymerizable material to yield a seconddiffractive waveguide for guiding light in a second visible wavelengthrange, wherein the second diffractive waveguide has a second yellownessindex and a second refractive index at the selected wavelength, wherein:a wavelength in the first visible wavelength range exceeds a wavelengthin the second visible wavelength range, the first refractive indexexceeds the second refractive index at the selected wavelength, and thefirst yellowness index exceeds the second yellowness index at theselected wavelength.
 16. The method of claim 15, further comprising:combining the first monomer and the second monomer in a third ratio toyield a third polymerizable material; and casting the thirdpolymerizable material in a third diffractive waveguide mold andpolymerizing the third polymerizable material to yield a thirddiffractive waveguide for guiding light in a third visible wavelengthrange, wherein the third diffractive waveguide has a third yellownessindex and a third refractive index at the selected wavelength, wherein:a wavelength in the second visible wavelength range exceeds a wavelengthin the third visible wavelength range, and the second yellowness indexexceeds the third yellowness index at the selected wavelength.
 17. Themethod of claim 16, wherein the second refractive index exceeds thethird refractive index at the selected wavelength.
 18. The method ofclaim 15, wherein the selected wavelength is 589 nm.
 19. A diffractivewaveguide stack comprising: a first diffractive waveguide for guidinglight in a first visible wavelength range, the first diffractivewaveguide comprising a first material and having a first refractiveindex at a selected wavelength and a first target refractive index at amidpoint of the first visible wavelength range; a second diffractivewaveguide for guiding light in a second visible wavelength range, thesecond diffractive waveguide comprising a second material and having asecond refractive index at the selected wavelength and a second targetrefractive index at a midpoint of the second visible wavelength range;and a third diffractive waveguide for guiding light in a third visiblewavelength range, the third diffractive waveguide comprising a thirdmaterial and having a third refractive index at the selected wavelengthand a third target refractive index at a midpoint of the third visiblewavelength range, wherein: the first visible wavelength range comprisesred light, the second visible wavelength range comprises green light,and the third visible wavelength range comprises blue light, and adifference between any two of the first target refractive index, thesecond target refractive index, and the third target refractive index atthe selected wavelength is less than 0.005 at the selected wavelength.20. The diffractive waveguide stack of claim 19, wherein the selectedwavelength is 589 nm.